All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
It is permissible for you to borrow your sister's phone and return it to her afterward. This does not count as assisting in sin, even if she uses it for impermissible matters.
Allah knows best.